The first hot-rolled straight-web steel sheet piles, also referred to as straight sheet piles, were already in use in the USA at the end of the 19th century. In Europe, these straight sheet piles have been rolled since the thirties of the 20th century. They comprise a straight web which lies in the wall axis and is delimited on each longitudinal side by an interlock strip. The individual straight sheet piles can be connected into a continuous sheet pile wall by means of these interlock strips.
Straight sheet piles are used particularly for the construction of cellular cofferdams without internal anchoring. Depending on the shape of the cells, a distinction is made between circular or straight cellular cofferdams. In the U.S.A., straight sheet piles have also been used for the construction of so called “open cells” (see, for example, U.S. Pat. No. 6,715,964). Closed and open cells are designed in such a way that the loads originating from the filling and water overpressure produce in the straight sheet piles only tensile stress in the direction of an horizontal wall axis.
In the dimensioning of the straight sheet piles for such cellular cofferdams, the stress (for example, the ring tensile force determined by means of the “boiler formula”) is compared with the sheet pile resistance. The latter is obtained, according to EN 1993-5, as the minimum arising from a failure in the interlock and a creep (i.e. a plastic deformation) in the web.
However, the manufacturer selects the steel quality for the web traditionally in such a way that the following condition is fulfilled:(fy·t)/S1>R/S2→fy>(R·S1)/(t·S2)  (1)in which:fy=the nominal yield point;t=the web thickness;R=the minimum interlock tensile strength guaranteed by the manufacturer (for example, R=5500 kN/m);S1=a safety coefficient for the creep in the web;S2=a safety coefficient for the failure in the interlock;the safety coefficients being different for the two types of failure, for example:S1=1.0 (creep in the web);S2=1.25 (failure in the interlock).
Adhering to the condition (1) given above ensures that the creep in the web will never be critical under tensile load on the straight sheet piles, that is to say that only the minimum interlock tensile strength R guaranteed by the manufacturer has to be respected. As a result, a failure of a straight sheet pile connection is almost always attributable to a breaking-open of an interlock connection.
A breaking-open of an interlock connection in the cell wall of a cofferdam cell causes a discontinuity in the absorption of the ring tensile forces. This results in a gap in the cell wall, which becomes enlarged and through which the soil filling of the cofferdam cell is flushed away. Without sufficient soil filling, however, the cofferdam cell can no longer withstand the loads originating from the water overpressure, which will inevitably result in a failure of the cofferdam.
Almost all straight sheet piles have symmetrical interlock strips of the “thumb and finger” type which, rotated through 180°, hook together with one another. In the case of two interlock strips locked together, the two thumbs engage one behind the other, the fingers respectively surrounding the thumb of the opposite interlock strip (see FIG. 1). A failure of such an interlock connection takes place either due to the tearing-off of the thumb subjected to tensile stress or due to the opening or breakage of the finger subjected to bending stress.
For reasons of cost, all manufacturers of straight sheet piles have in their standard delivery range only three to four straight sheet piles which differ from one another essentially in the thickness of their web. As a rule, web thicknesses of 11 to 13 mm are implemented in such straight sheet piles. The selection of the steel quality then determines the minimum interlock tensile strength of the web, wherein, as a rule, values of 2000-4000 kN/m being ensured. New high-strength steels, such as, for example, the steel S 460 GP, make it possible to ensure even a minimum interlock tensile strength of 5500 kN/m. Since an increased steel quality also leads to an increase in the yield point in the web, it is always warranted that the condition (1) remains fulfilled. It will also be appreciated in this context that straight sheet piles with a thicker web have, as a rule, a higher minimum interlock tensile strength, since, during the rolling of a thicker web, the parts of the interlock which are critical for the interlock tensile strength can also be rolled more thickly.
It sometime happens that the manufacturer cannot achieve the minimum interlock tensile strength required for the construction project with straight sheet piles from the standard delivery program. For reasons of cost, however, a manufacturer is hardly prepared to roll special straight sheet piles for individual construction projects. In such instances, it is known to increase the minimum interlock tensile strength of webs from the standard delivery program in that, starting from an existing calibration, the “calibre” is opened further during the rolling operation, that is to say the gap set between the upper and lower roll is slightly increased. As a result, not only does the web become slightly thicker, but the parts of the interlock which are critical for the interlock tensile strength are also of stronger design and consequently afford higher resistance. Such a method is described, for example, in JP55138511. It should be noted that this procedure also ensures that the condition (1) remains fulfilled.
For the purpose of increasing the interlock tensile strength, it has likewise been proposed to vary the geometry of the interlock strips (see, for example, JP56020227). However, for this purpose the manufacturer would have to invest in new rolls. Furthermore, he would subsequently have to include in his delivery program two different interlock types for straight sheet piles, which does not exactly simplify the logistics. For both reasons, the manufacturers of straight sheet piles are therefore hardly prepared to follow this path.
It has also been known for a long time that straight sheet piles may also be exposed to high dynamic loads in specific cofferdams. The walls of the cells are, for example, rammed by ships and, in the case of spring tides and storm tides, are exposed to the impact of heavy drift flotsam. Moreover, many cofferdams are also erected in earthquake zones. For such dynamic load situations, the straight sheet piles would actually have to be designed in a completely different way from hitherto. Thus, for example, it would have to be ensured that the straight sheet piles can absorb substantially higher deformation energy than hitherto before the failure of an interlock connection occurs. However, since it has been assumed that major investments are required for the production of such a completely new straight sheet pile, no manufacturer has hitherto put on the market a straight sheet pile which is designed particularly for the dynamic load situations mentioned above.
The present invention is based on the surprising finding that a straight sheet pile from the standard delivery range of a manufacturer can be modified at very low outlay in such a way that it is substantially more suitable for the absorption of dynamic stresses.